9be4c94204
This makes it possible to do a border render inside a viewport even when not looking through the camera. Render border could be defined by Ctrl-B shortcut (works for both camera render border and viewport render border). Camera render border could still be defined using Shift-B (so no muscule memory would be broken). Currently used a special flag of operator to do this, otherwise you'll need to either two operators with different poll callback or it could go into conflict with a border zoom, Border render of a viewport could be enabled/disabled in View panel using "Render Border" option.